Scanning Electron Microscopy

Overview

Scanning Electron Microscopy (SEM) utilizes a high energy beam of focused electrons to generate images or micrographs of the topographic detail and/or material variation in small specimens. SEMs are capable of few-nanometer resolution with a well aligned beam and appropriate sample preparation and operating conditions. Images are formed by scanning an electron beam point-by-point across a specimen surface, which generates various signals that are collected, converted to an intensity grayscale, and correlated to a location for display on the monitor.

Equipment

• JEOL 7500F HRSEM | 7500F Reference Guide Image with secondary electrons, backscattered electrons, and transmission electrons at high vacuum and high resolution, even at very low voltages. Elemental analysis with x-rays available with EDS by EDAX.
• TFS Quanta 600 FEG ESEM | Quanta Reference Guide Image non-conductive or vacuum-sensitive specimens in high vacuum or in a low vacuum water vapor environment. EDAX EDS elemental analysis and EBSD crystallography available. This microscope is equipped with a large chamber for in-situ imaging and high- and low-temperature stages.

Techniques

• EDS
• EBSD
• Low Voltage SEM
• Low Vacuum SEM

Key Principles

Conductivity

Most SEMs perform best when imaging conductive samples. Conductive samples allow electrons from the beam to travel freely to ground if they are not emitted as signal. Non-conductive samples may be sputter coated with a few nanometers of conductive material to improve images.

Exceptions include: Low Vacuum SEM and Low Voltage SEM which compensate for the lack of conductivity with either charge balance and signal amplification via ionized water vapor or reduced incident electron energy.

Scanning

SEM images are generated one pixel at a time as the electron beam rasters, or scans, across a specified region. Slower scans with longer dwell times result in more signal, but may introduce artifacts from charge buildup. Short scans tend to appear very noisy due to a low signal-to-noise ratio (SNR.)

Beam-Sample Interactions

Incident electrons are either scattered or absorbed by the sample.

Absorbed electrons travel through conductive specimens through the stub, sample holder, and stage to ground. However, these electrons may appear as charge buildup in less conductive specimens, or specimens not well grounded to the stub or sample holder.

Scattered electrons produce signal.

• Secondary electrons encounter multiple inelastic scattering events before escaping through the specimen surface. These yield topographic contrast and come from the first few nanometers of the sample surface.
• Backscattered electrons elastically scatter once before escaping the surface. These yield material contrast and are generated from up to 100nm below the specimen surface.
• Characteristic X-Rays are emitted when a higher energy electron falls to the energy level of an electron displaced by the incident beam. These x-rays can identify specific elements and may be generated from up to 1um beneath the specimen surface. Characteristic x-rays are fundamental to EDS analysis.

Signal Collection

Emitted signals (secondary electrons, backscattered electrons, characteristic x-rays) are each collected by dedicated detectors, which may have a bias applied. SEM detectors collect all the signal in the chamber for a given point, and computer controls correlate the pixel of the image with the scan location of the beam. Images may appear to have shadows if the path between the point of interaction and the detector is blocked by another portion of the specimen topography.

Troubleshooting

Achieving a sharp image in SEM takes practice, and is not always due to the focus. It is important to ask the right questions when troubleshooting to find the best solution to improve image quality:

Are my beam settings appropriate for this sample?

Every sample interacts with the electron beam differently. Consider:

• What part of the sample do I want to see?
• Is the sample charging?

You may need to change the beam voltage or probe current/spot size. There will always be a balance between beam energy, specimen damage, charging, and image quality. Try changing one setting at a time in either direction (higher/lower, larger/smaller) and observe how your sample responds.

Have I aligned the beam?

There are three key steps to aligning an SEM:

• Focus: The focal plane should be on the sample surface.
• Aperture/Lens alignment: The beam should straight from the pole piece, and so that the brightest, most central part of the beam is selected by the aperture.
• Stigmators: The cross section of the beam both in focus and out of focus should be circular, yielding the smallest probe diameter at focus.

If you are having trouble focusing, the number one reason is that the stigmators are not aligned. Try aligning them at a higher magnification and vary the focus (in and out) until the image is sharpest at focus and does not stretch or smear directionally on either side of focus.

Is it how I'm collecting signal?

SEMs collect signal with a single-point detector, not an array like the detector in a camera. Because of this, we must consider:

• Beam-Specimen Interactions, or how the beam generates signal.
• How signal reaches the detector, or the position of the detector relative to the specimen.
• How images are formed, including scanning rates and image size or pixel resolution.

In some cases, slowing the scan speed or integrating more frames of a faster speed can improve the signal-to-noise ratio (SNR) in response to different types of charging. In other, the sample may need to be moved to a shorter working distance to improve contrast. Be careful if moving the stage closer to the pole piece: use the chamberscope, be aware of stage limits, and ask for help.

Do I need to rethink my sample preparation?

Sample preparation can help or impede signal generation. Generally speaking, well prepared samples produce better images. Consider:

• How the sample itself is prepared.
• How the sample is secured to the stub.

With some exceptions for low-vacuum SEM, ensure there is a conductive pathway between the specimen surface and the stub. This may mean coating, adding copper tape or a conductive clip, pressing specimens better to carbon tape, using a conductive substrate, or ensuring powders or fibers are adhered in as thin a layer as possible. Remember: what may seem small to us, may still seem large to the SEM. Failure to secure or ground specimens well may result in charging or drift.

Remember that SEM is a skill and every sample is different - take time to practice and observe how images change in response to beam settings and alignments.